Similar to a general diode, a light emitting diode (LED) has a characteristic of emitting light when a certain condition is satisfied, namely, when a forward voltage is applied and the magnitude of the applied voltage is equal to or greater than that of a threshold voltage, the LED is turned on and current flows in the light-emitting diode, whereby light is emitted.
An AC direct-type light-emitting diode, which is directly driven by the voltage of an AC power source, (hereinafter, referred to ‘power voltage’) is connected to a full-wave rectifier, and in this case, one or more light-emitting diodes are connected in series, or in series-parallel, which is the combination of series and parallel connection structures.
As described above, when a voltage greater than a turn-on voltage is applied, a light-emitting diode is turned on and a current flows through the light-emitting diode, whereas a voltage less than the turn-on voltage is applied, the light-emitting diode is turned off and a current does not flow in the diode.
Accordingly, based on one power voltage cycle, the turn-on time of the light-emitting diode is short, thus, the amount of light emitted from the light-emitting diode decreases and total harmonic distortion occurs.
When the number of series-connected light-emitting diodes increases, the voltage required for turning on the light-emitting diodes also increases and the turn-on time of the light-emitting diodes is shorter. The reduction in the amount of the emitted light and total harmonic distortion become severe, and it may lead to the increase in manufacturing cost.
Conversely, when the number of series-connected light-emitting diodes decreases, the voltage for driving the light-emitting diodes decreases but an overcurrent flows in the light-emitting diodes. The life of the light-emitting diode is greatly decreased, and overcurrent occurs depending on the variation of the AC voltage.
Therefore, it is necessary to develop a light-emitting diode driving apparatus that is not affected by variation of power voltage, increases the amount of emitted light, prevents overcurrent, and decreases the manufacturing cost.
FIG. 1 illustrates a light-emitting driving circuit in which an AC power source AC, a rectification diode part Dr, a current regulating resistor Rr, and a light-emitting diode part De that has a plurality of serial-connected light-emitting diodes are connected in series.
FIG. 2 illustrates the waveform of the voltage VAC of the AC power (hereinafter, referred to ‘power voltage’) applied in FIG. 1, the waveform of the current IAC of the AC power, the waveform of the rectified voltage VCC rectified by the rectification diode part Dr, and the waveform of the rectified current ICC that flows in the light-emitting diode part De.
As shown FIG. 1, when the power voltage VAC passes through the rectification diode part Dr, the power voltage is full-wave rectified, and the full-wave rectified voltage VCC is applied to the light-emitting diode part De via the resistor Rr.
When the magnitude of the rectified voltage VCC is equal to or less than the total forward threshold voltage Vth1 of the light-emitting diode part De that has a plurality of series-connected light-emitting diodes, (namely, the total of the forward threshold voltages of the light-emitting diodes each), the light-emitting diode part De is turned off during a certain time (t1, t3) and a current ICC does not flow in the light-emitting diode part De, as shown in FIG. 2.
However, when the magnitude of the rectified voltage VCC is greater than the forward threshold voltage Vth1 (t2), the light-emitting diode part De is turned on and the current Icc starts to flow through the light-emitting diode part De. In this case, the magnitude of the current ICC corresponds to a value that is obtained by dividing the difference between the rectified voltage VCC and the forward threshold voltage Vth1 by the resistance of the resistor Rr. Therefore, if the rectified voltage VCC increases, there is a problem that the current flowing in the light-emitting diode part De becomes greater than a maximum allowable current.
Because the magnitude of the voltage required for the turn-on operation of the light-emitting diode part De, namely, the magnitude of the forward threshold voltage Vth increases with the number of series-connected light-emitting diodes, the turn-on time of the light-emitting diode part De is shorter.
Accordingly, the magnitude of total harmonic distortion increases and the amount of light emitted from the light-emitting diode part De decreases.
When the forward threshold voltage Vth1 of the light-emitting diode part De decreases or when the power voltage VAC increases, a current greater than an allowable current may flow in the light-emitting diode part De, thus the life of the light-emitting diode part De is reduced and the reliability of the operation of the light-emitting diode part is decreased.
Globally, total harmonic distortion that causes various electrical noises is the target of regulations, and when the amount of the emitted light of the light-emitting diode part De is reduced, more light-emitting diodes must be used to compensate for the reduced amount of the emitted light, thus the manufacturing cost of a lighting device having the light-emitting diodes is increased.
Next, FIG. 3 illustrates a light-emitting diode driving circuit for improving total harmonic distortion.
Referring to FIG. 3, a power source AC is connected in series with a current regulating resistor R, a first light-emitting diode part Da, and a second light-emitting diode part Db. The first light-emitting diode part Da has two light-emitting diodes Da1 and Da2, which are connected in anti-parallel, and the second light-emitting diode part Db also has two light-emitting diodes Db1 and Db2, which are connected in anti-parallel.
In the light-emitting diode driving circuit illustrated in FIG. 3, in order to improve total harmonic distortion, a capacitor C1 is connected to a connection point na between the resistor R and the first light-emitting diode part Da, and to a connection point nb between the first light-emitting diode part Da and the second light-emitting diode part Db; and the power voltage VAC is connected to the first and second light-emitting diode parts Da and Db via resistor R, without a rectifier.
FIG. 4 illustrates the waveforms of the power voltage VAC and the current IAC, which are applied to the first and second light-emitting diode parts Da and Db of FIG. 3, and the waveform of a voltage VR that passes through the resistor R, the waveform of a current IDa flowing in the first light-emitting diode part Da, and the waveform of a current IDb flowing in the second light-emitting diode part Db.
When the magnitude of the voltage VR is less than a forward threshold voltage Vth2, a current does not flow in the first light-emitting diode part Da, and when the magnitude of the voltage VR is equal to or greater than the total forward threshold voltage Vth2, the current IDa2 flows in the forward light-emitting diode Da2 of the first light-emitting diode part Da during a positive (+) half cycle of the power voltage VAC, and the current IDa1 flows in the backward light-emitting diode Da1 of the first light-emitting diode part Da during a negative (−) half cycle of the power voltage VAC, whereby they form the current IDa of the first light-emitting diode part.
If the magnitude of the voltage VR is less than the forward threshold voltage Vth2, a charging current flows in the second light-emitting diode part Db through the capacitor C1 while the power voltage VAC increases in a positive (+) direction, and a discharging current flows also through the capacitor C1 while the power voltage VAC decreases in a negative (−) direction.
When the magnitude of the voltage VR is equal to or greater than the forward threshold voltage Vth2, the current IDb2 flows in the forward light-emitting diode Db2 of the second light-emitting diode part Db via the forward light-emitting diode Da2 of the first light-emitting diode part Da during a positive (+) half cycle of the power voltage VAC, and the current IDb1 flows in the backward light-emitting diode Db1 of the second light-emitting diode part Db via the backward light-emitting diode Da1 of the first light-emitting diode part Da during a negative (−) half cycle of the power voltage VAC, whereby they form the current IDb of the second light-emitting diode part.
However, when the power voltage VAC decreases, a current does not flow through the capacitor C1, and when the magnitude of the voltage VR is less than the forward threshold voltage Vth2, a current does not flow in the second light-emitting diode part Db such as in the first light-emitting diode part Da.
When the power voltage VAC increases, a charging-discharging current is generated in the capacitor C1 through the second light-emitting diode part Db, whereby total harmonic distortion of the current IAC that flows in the power voltage VAC may be improved to a certain degree. However, because the capacitor C1 has the short life and should tolerate a high voltage, the cost of the capacitor C1 may be increased and it is difficult to reduce the size of a product due to the size of the capacitor C1.
Also, because the charging-discharging current of the capacitor C1 flows only in the second light-emitting diode part Db, which corresponds to the half of the used light-emitting diodes, to reduce the total harmonic distortion, the current flowing in the second light-emitting diode part Db increases and is higher than the current flowing in the first light-emitting diode part Da. When a current is maximally provided to the first light-emitting diode part Da, an overcurrent flows in the second light-emitting diode part Db. Therefore, a current that is enough to drive the first and second light-emitting diode parts Da and Db may not be provided to the light-emitting diode driving circuit.
Consequently, a maximum allowable current may not flow in the first and second light-emitting diode parts Da and Db, thus, the amount of emitted light is reduced.
Also, if the power voltage VAC increases due to the variation of the voltage, the magnitude of the current flowing in the first and second light-emitting diode parts Da and Db becomes greater than the maximum allowable current. Therefore, in consideration of the variation of the power voltage VAC, the current flowing in the light-emitting diode may not reach the maximum allowable current, thus the amount of the emitted light is reduced.